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diff --git a/js/src/vm/HelperThreads.h b/js/src/vm/HelperThreads.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
+ * vim: set ts=8 sts=4 et sw=4 tw=99:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/*
+ * Definitions for managing off-main-thread work using a process wide list
+ * of worklist items and pool of threads. Worklist items are engine internal,
+ * and are distinct from e.g. web workers.
+ */
+
+#ifndef vm_HelperThreads_h
+#define vm_HelperThreads_h
+
+#include "mozilla/GuardObjects.h"
+#include "mozilla/PodOperations.h"
+#include "mozilla/TimeStamp.h"
+#include "mozilla/Variant.h"
+
+#include "jscntxt.h"
+
+#include "frontend/TokenStream.h"
+#include "jit/Ion.h"
+#include "threading/ConditionVariable.h"
+#include "vm/MutexIDs.h"
+
+namespace JS {
+struct Zone;
+} // namespace JS
+
+namespace js {
+
+class AutoLockHelperThreadState;
+class AutoUnlockHelperThreadState;
+class PromiseTask;
+struct HelperThread;
+struct ParseTask;
+namespace jit {
+  class IonBuilder;
+} // namespace jit
+namespace wasm {
+  class FuncIR;
+  class FunctionCompileResults;
+  class IonCompileTask;
+  typedef Vector<IonCompileTask*, 0, SystemAllocPolicy> IonCompileTaskPtrVector;
+} // namespace wasm
+
+enum class ParseTaskKind
+{
+    Script,
+    Module
+};
+
+// Per-process state for off thread work items.
+class GlobalHelperThreadState
+{
+    friend class AutoLockHelperThreadState;
+    friend class AutoUnlockHelperThreadState;
+
+  public:
+    // Number of CPUs to treat this machine as having when creating threads.
+    // May be accessed without locking.
+    size_t cpuCount;
+
+    // Number of threads to create. May be accessed without locking.
+    size_t threadCount;
+
+    typedef Vector<jit::IonBuilder*, 0, SystemAllocPolicy> IonBuilderVector;
+    typedef Vector<ParseTask*, 0, SystemAllocPolicy> ParseTaskVector;
+    typedef Vector<SourceCompressionTask*, 0, SystemAllocPolicy> SourceCompressionTaskVector;
+    typedef Vector<GCHelperState*, 0, SystemAllocPolicy> GCHelperStateVector;
+    typedef Vector<GCParallelTask*, 0, SystemAllocPolicy> GCParallelTaskVector;
+    typedef Vector<PromiseTask*, 0, SystemAllocPolicy> PromiseTaskVector;
+
+    // List of available threads, or null if the thread state has not been initialized.
+    using HelperThreadVector = Vector<HelperThread, 0, SystemAllocPolicy>;
+    UniquePtr<HelperThreadVector> threads;
+
+  private:
+    // The lists below are all protected by |lock|.
+
+    // Ion compilation worklist and finished jobs.
+    IonBuilderVector ionWorklist_, ionFinishedList_;
+
+    // wasm worklist and finished jobs.
+    wasm::IonCompileTaskPtrVector wasmWorklist_, wasmFinishedList_;
+
+  public:
+    // For now, only allow a single parallel wasm compilation to happen at a
+    // time. This avoids race conditions on wasmWorklist/wasmFinishedList/etc.
+    mozilla::Atomic<bool> wasmCompilationInProgress;
+
+  private:
+    // Async tasks that, upon completion, are dispatched back to the JSContext's
+    // owner thread via embedding callbacks instead of a finished list.
+    PromiseTaskVector promiseTasks_;
+
+    // Script parsing/emitting worklist and finished jobs.
+    ParseTaskVector parseWorklist_, parseFinishedList_;
+
+    // Parse tasks waiting for an atoms-zone GC to complete.
+    ParseTaskVector parseWaitingOnGC_;
+
+    // Source compression worklist.
+    SourceCompressionTaskVector compressionWorklist_;
+
+    // Runtimes which have sweeping / allocating work to do.
+    GCHelperStateVector gcHelperWorklist_;
+
+    // GC tasks needing to be done in parallel.
+    GCParallelTaskVector gcParallelWorklist_;
+
+    ParseTask* removeFinishedParseTask(ParseTaskKind kind, void* token);
+
+  public:
+    size_t maxIonCompilationThreads() const;
+    size_t maxUnpausedIonCompilationThreads() const;
+    size_t maxWasmCompilationThreads() const;
+    size_t maxParseThreads() const;
+    size_t maxCompressionThreads() const;
+    size_t maxGCHelperThreads() const;
+    size_t maxGCParallelThreads() const;
+
+    GlobalHelperThreadState();
+
+    bool ensureInitialized();
+    void finish();
+    void finishThreads();
+
+    void lock();
+    void unlock();
+
+    enum CondVar {
+        // For notifying threads waiting for work that they may be able to make progress.
+        CONSUMER,
+
+        // For notifying threads doing work that they may be able to make progress.
+        PRODUCER,
+
+        // For notifying threads doing work which are paused that they may be
+        // able to resume making progress.
+        PAUSE
+    };
+
+    void wait(AutoLockHelperThreadState& locked, CondVar which,
+              mozilla::TimeDuration timeout = mozilla::TimeDuration::Forever());
+    void notifyAll(CondVar which, const AutoLockHelperThreadState&);
+    void notifyOne(CondVar which, const AutoLockHelperThreadState&);
+
+    // Helper method for removing items from the vectors below while iterating over them.
+    template <typename T>
+    void remove(T& vector, size_t* index)
+    {
+        vector[(*index)--] = vector.back();
+        vector.popBack();
+    }
+
+    IonBuilderVector& ionWorklist(const AutoLockHelperThreadState&) {
+        return ionWorklist_;
+    }
+    IonBuilderVector& ionFinishedList(const AutoLockHelperThreadState&) {
+        return ionFinishedList_;
+    }
+
+    wasm::IonCompileTaskPtrVector& wasmWorklist(const AutoLockHelperThreadState&) {
+        return wasmWorklist_;
+    }
+    wasm::IonCompileTaskPtrVector& wasmFinishedList(const AutoLockHelperThreadState&) {
+        return wasmFinishedList_;
+    }
+
+    PromiseTaskVector& promiseTasks(const AutoLockHelperThreadState&) {
+        return promiseTasks_;
+    }
+
+    ParseTaskVector& parseWorklist(const AutoLockHelperThreadState&) {
+        return parseWorklist_;
+    }
+    ParseTaskVector& parseFinishedList(const AutoLockHelperThreadState&) {
+        return parseFinishedList_;
+    }
+    ParseTaskVector& parseWaitingOnGC(const AutoLockHelperThreadState&) {
+        return parseWaitingOnGC_;
+    }
+
+    SourceCompressionTaskVector& compressionWorklist(const AutoLockHelperThreadState&) {
+        return compressionWorklist_;
+    }
+
+    GCHelperStateVector& gcHelperWorklist(const AutoLockHelperThreadState&) {
+        return gcHelperWorklist_;
+    }
+
+    GCParallelTaskVector& gcParallelWorklist(const AutoLockHelperThreadState&) {
+        return gcParallelWorklist_;
+    }
+
+    bool canStartWasmCompile(const AutoLockHelperThreadState& lock);
+    bool canStartPromiseTask(const AutoLockHelperThreadState& lock);
+    bool canStartIonCompile(const AutoLockHelperThreadState& lock);
+    bool canStartParseTask(const AutoLockHelperThreadState& lock);
+    bool canStartCompressionTask(const AutoLockHelperThreadState& lock);
+    bool canStartGCHelperTask(const AutoLockHelperThreadState& lock);
+    bool canStartGCParallelTask(const AutoLockHelperThreadState& lock);
+
+    // Unlike the methods above, the value returned by this method can change
+    // over time, even if the helper thread state lock is held throughout.
+    bool pendingIonCompileHasSufficientPriority(const AutoLockHelperThreadState& lock);
+
+    jit::IonBuilder* highestPriorityPendingIonCompile(const AutoLockHelperThreadState& lock,
+                                                      bool remove = false);
+    HelperThread* lowestPriorityUnpausedIonCompileAtThreshold(
+        const AutoLockHelperThreadState& lock);
+    HelperThread* highestPriorityPausedIonCompile(const AutoLockHelperThreadState& lock);
+
+    uint32_t harvestFailedWasmJobs(const AutoLockHelperThreadState&) {
+        uint32_t n = numWasmFailedJobs;
+        numWasmFailedJobs = 0;
+        return n;
+    }
+    void noteWasmFailure(const AutoLockHelperThreadState&) {
+        // Be mindful to signal the main thread after calling this function.
+        numWasmFailedJobs++;
+    }
+    bool wasmFailed(const AutoLockHelperThreadState&) {
+        return bool(numWasmFailedJobs);
+    }
+
+    JSScript* finishParseTask(JSContext* cx, ParseTaskKind kind, void* token);
+    void cancelParseTask(JSContext* cx, ParseTaskKind kind, void* token);
+
+    void mergeParseTaskCompartment(JSContext* cx, ParseTask* parseTask,
+                                   Handle<GlobalObject*> global,
+                                   JSCompartment* dest);
+
+    void trace(JSTracer* trc);
+
+  private:
+    /*
+     * Number of wasm jobs that encountered failure for the active module.
+     * Their parent is logically the main thread, and this number serves for harvesting.
+     */
+    uint32_t numWasmFailedJobs;
+
+  public:
+    JSScript* finishScriptParseTask(JSContext* cx, void* token);
+    JSObject* finishModuleParseTask(JSContext* cx, void* token);
+    bool compressionInProgress(SourceCompressionTask* task, const AutoLockHelperThreadState& lock);
+    SourceCompressionTask* compressionTaskForSource(ScriptSource* ss, const AutoLockHelperThreadState& lock);
+
+    bool hasActiveThreads(const AutoLockHelperThreadState&);
+    void waitForAllThreads();
+
+    template <typename T>
+    bool checkTaskThreadLimit(size_t maxThreads) const;
+
+  private:
+
+    /*
+     * Lock protecting all mutable shared state accessed by helper threads, and
+     * used by all condition variables.
+     */
+    js::Mutex helperLock;
+
+    /* Condvars for threads waiting/notifying each other. */
+    js::ConditionVariable consumerWakeup;
+    js::ConditionVariable producerWakeup;
+    js::ConditionVariable pauseWakeup;
+
+    js::ConditionVariable& whichWakeup(CondVar which) {
+        switch (which) {
+          case CONSUMER: return consumerWakeup;
+          case PRODUCER: return producerWakeup;
+          case PAUSE: return pauseWakeup;
+          default: MOZ_CRASH("Invalid CondVar in |whichWakeup|");
+        }
+    }
+};
+
+static inline GlobalHelperThreadState&
+HelperThreadState()
+{
+    extern GlobalHelperThreadState* gHelperThreadState;
+
+    MOZ_ASSERT(gHelperThreadState);
+    return *gHelperThreadState;
+}
+
+/* Individual helper thread, one allocated per core. */
+struct HelperThread
+{
+    mozilla::Maybe<PerThreadData> threadData;
+    mozilla::Maybe<Thread> thread;
+
+    /*
+     * Indicate to a thread that it should terminate itself. This is only read
+     * or written with the helper thread state lock held.
+     */
+    bool terminate;
+
+    /*
+     * Indicate to a thread that it should pause execution. This is only
+     * written with the helper thread state lock held, but may be read from
+     * without the lock held.
+     */
+    mozilla::Atomic<bool, mozilla::Relaxed> pause;
+
+    /* The current task being executed by this thread, if any. */
+    mozilla::Maybe<mozilla::Variant<jit::IonBuilder*,
+                                    wasm::IonCompileTask*,
+                                    PromiseTask*,
+                                    ParseTask*,
+                                    SourceCompressionTask*,
+                                    GCHelperState*,
+                                    GCParallelTask*>> currentTask;
+
+    bool idle() const {
+        return currentTask.isNothing();
+    }
+
+    /* Any builder currently being compiled by Ion on this thread. */
+    jit::IonBuilder* ionBuilder() {
+        return maybeCurrentTaskAs<jit::IonBuilder*>();
+    }
+
+    /* Any wasm data currently being optimized on this thread. */
+    wasm::IonCompileTask* wasmTask() {
+        return maybeCurrentTaskAs<wasm::IonCompileTask*>();
+    }
+
+    /* Any source being parsed/emitted on this thread. */
+    ParseTask* parseTask() {
+        return maybeCurrentTaskAs<ParseTask*>();
+    }
+
+    /* Any source being compressed on this thread. */
+    SourceCompressionTask* compressionTask() {
+        return maybeCurrentTaskAs<SourceCompressionTask*>();
+    }
+
+    /* Any GC state for background sweeping or allocating being performed. */
+    GCHelperState* gcHelperTask() {
+        return maybeCurrentTaskAs<GCHelperState*>();
+    }
+
+    /* State required to perform a GC parallel task. */
+    GCParallelTask* gcParallelTask() {
+        return maybeCurrentTaskAs<GCParallelTask*>();
+    }
+
+    void destroy();
+
+    static void ThreadMain(void* arg);
+    void threadLoop();
+
+  private:
+    template <typename T>
+    T maybeCurrentTaskAs() {
+        if (currentTask.isSome() && currentTask->is<T>())
+            return currentTask->as<T>();
+
+        return nullptr;
+    }
+
+    void handleWasmWorkload(AutoLockHelperThreadState& locked);
+    void handlePromiseTaskWorkload(AutoLockHelperThreadState& locked);
+    void handleIonWorkload(AutoLockHelperThreadState& locked);
+    void handleParseWorkload(AutoLockHelperThreadState& locked, uintptr_t stackLimit);
+    void handleCompressionWorkload(AutoLockHelperThreadState& locked);
+    void handleGCHelperWorkload(AutoLockHelperThreadState& locked);
+    void handleGCParallelWorkload(AutoLockHelperThreadState& locked);
+};
+
+/* Methods for interacting with helper threads. */
+
+// Create data structures used by helper threads.
+bool
+CreateHelperThreadsState();
+
+// Destroy data structures used by helper threads.
+void
+DestroyHelperThreadsState();
+
+// Initialize helper threads unless already initialized.
+bool
+EnsureHelperThreadsInitialized();
+
+// This allows the JS shell to override GetCPUCount() when passed the
+// --thread-count=N option.
+void
+SetFakeCPUCount(size_t count);
+
+// Pause the current thread until it's pause flag is unset.
+void
+PauseCurrentHelperThread();
+
+/* Perform MIR optimization and LIR generation on a single function. */
+bool
+StartOffThreadWasmCompile(wasm::IonCompileTask* task);
+
+/*
+ * If helper threads are available, start executing the given PromiseTask on a
+ * helper thread, finishing back on the originating JSContext's owner thread. If
+ * no helper threads are available, the PromiseTask is synchronously executed
+ * and finished.
+ */
+bool
+StartPromiseTask(JSContext* cx, UniquePtr<PromiseTask> task);
+
+/*
+ * Schedule an Ion compilation for a script, given a builder which has been
+ * generated and read everything needed from the VM state.
+ */
+bool
+StartOffThreadIonCompile(JSContext* cx, jit::IonBuilder* builder);
+
+struct AllCompilations {};
+struct ZonesInState { JSRuntime* runtime; JS::Zone::GCState state; };
+
+using CompilationSelector = mozilla::Variant<JSScript*,
+                                             JSCompartment*,
+                                             ZonesInState,
+                                             JSRuntime*,
+                                             AllCompilations>;
+
+/*
+ * Cancel scheduled or in progress Ion compilations.
+ */
+void
+CancelOffThreadIonCompile(CompilationSelector selector, bool discardLazyLinkList);
+
+inline void
+CancelOffThreadIonCompile(JSScript* script)
+{
+    CancelOffThreadIonCompile(CompilationSelector(script), true);
+}
+
+inline void
+CancelOffThreadIonCompile(JSCompartment* comp)
+{
+    CancelOffThreadIonCompile(CompilationSelector(comp), true);
+}
+
+inline void
+CancelOffThreadIonCompile(JSRuntime* runtime, JS::Zone::GCState state)
+{
+    CancelOffThreadIonCompile(CompilationSelector(ZonesInState{runtime, state}), true);
+}
+
+inline void
+CancelOffThreadIonCompile(JSRuntime* runtime)
+{
+    CancelOffThreadIonCompile(CompilationSelector(runtime), true);
+}
+
+inline void
+CancelOffThreadIonCompile()
+{
+    CancelOffThreadIonCompile(CompilationSelector(AllCompilations()), false);
+}
+
+#ifdef DEBUG
+bool
+HasOffThreadIonCompile(JSCompartment* comp);
+#endif
+
+/* Cancel all scheduled, in progress or finished parses for runtime. */
+void
+CancelOffThreadParses(JSRuntime* runtime);
+
+/*
+ * Start a parse/emit cycle for a stream of source. The characters must stay
+ * alive until the compilation finishes.
+ */
+bool
+StartOffThreadParseScript(JSContext* cx, const ReadOnlyCompileOptions& options,
+                          const char16_t* chars, size_t length,
+                          JS::OffThreadCompileCallback callback, void* callbackData);
+
+bool
+StartOffThreadParseModule(JSContext* cx, const ReadOnlyCompileOptions& options,
+                          const char16_t* chars, size_t length,
+                          JS::OffThreadCompileCallback callback, void* callbackData);
+
+/*
+ * Called at the end of GC to enqueue any Parse tasks that were waiting on an
+ * atoms-zone GC to finish.
+ */
+void
+EnqueuePendingParseTasksAfterGC(JSRuntime* rt);
+
+struct AutoEnqueuePendingParseTasksAfterGC {
+    const gc::GCRuntime& gc_;
+    explicit AutoEnqueuePendingParseTasksAfterGC(const gc::GCRuntime& gc) : gc_(gc) {}
+    ~AutoEnqueuePendingParseTasksAfterGC();
+};
+
+/* Start a compression job for the specified token. */
+bool
+StartOffThreadCompression(ExclusiveContext* cx, SourceCompressionTask* task);
+
+class MOZ_RAII AutoLockHelperThreadState : public LockGuard<Mutex>
+{
+    using Base = LockGuard<Mutex>;
+
+    MOZ_DECL_USE_GUARD_OBJECT_NOTIFIER
+
+  public:
+    explicit AutoLockHelperThreadState(MOZ_GUARD_OBJECT_NOTIFIER_ONLY_PARAM)
+      : Base(HelperThreadState().helperLock)
+    {
+        MOZ_GUARD_OBJECT_NOTIFIER_INIT;
+    }
+};
+
+class MOZ_RAII AutoUnlockHelperThreadState : public UnlockGuard<Mutex>
+{
+    using Base = UnlockGuard<Mutex>;
+
+    MOZ_DECL_USE_GUARD_OBJECT_NOTIFIER
+
+  public:
+
+    explicit AutoUnlockHelperThreadState(AutoLockHelperThreadState& locked
+                                         MOZ_GUARD_OBJECT_NOTIFIER_PARAM)
+      : Base(locked)
+    {
+        MOZ_GUARD_OBJECT_NOTIFIER_INIT;
+    }
+};
+
+struct ParseTask
+{
+    ParseTaskKind kind;
+    ExclusiveContext* cx;
+    OwningCompileOptions options;
+    const char16_t* chars;
+    size_t length;
+    LifoAlloc alloc;
+
+    // Rooted pointer to the global object used by 'cx'.
+    JSObject* exclusiveContextGlobal;
+
+    // Callback invoked off the main thread when the parse finishes.
+    JS::OffThreadCompileCallback callback;
+    void* callbackData;
+
+    // Holds the final script between the invocation of the callback and the
+    // point where FinishOffThreadScript is called, which will destroy the
+    // ParseTask.
+    JSScript* script;
+
+    // Holds the ScriptSourceObject generated for the script compilation.
+    ScriptSourceObject* sourceObject;
+
+    // Any errors or warnings produced during compilation. These are reported
+    // when finishing the script.
+    Vector<frontend::CompileError*> errors;
+    bool overRecursed;
+    bool outOfMemory;
+
+    ParseTask(ParseTaskKind kind, ExclusiveContext* cx, JSObject* exclusiveContextGlobal,
+              JSContext* initCx, const char16_t* chars, size_t length,
+              JS::OffThreadCompileCallback callback, void* callbackData);
+    bool init(JSContext* cx, const ReadOnlyCompileOptions& options);
+
+    void activate(JSRuntime* rt);
+    virtual void parse() = 0;
+    bool finish(JSContext* cx);
+
+    bool runtimeMatches(JSRuntime* rt) {
+        return cx->runtimeMatches(rt);
+    }
+
+    virtual ~ParseTask();
+
+    void trace(JSTracer* trc);
+};
+
+struct ScriptParseTask : public ParseTask
+{
+    ScriptParseTask(ExclusiveContext* cx, JSObject* exclusiveContextGlobal,
+                    JSContext* initCx, const char16_t* chars, size_t length,
+                    JS::OffThreadCompileCallback callback, void* callbackData);
+    void parse() override;
+};
+
+struct ModuleParseTask : public ParseTask
+{
+    ModuleParseTask(ExclusiveContext* cx, JSObject* exclusiveContextGlobal,
+                    JSContext* initCx, const char16_t* chars, size_t length,
+                    JS::OffThreadCompileCallback callback, void* callbackData);
+    void parse() override;
+};
+
+// Return whether, if a new parse task was started, it would need to wait for
+// an in-progress GC to complete before starting.
+extern bool
+OffThreadParsingMustWaitForGC(JSRuntime* rt);
+
+// Compression tasks are allocated on the stack by their triggering thread,
+// which will block on the compression completing as the task goes out of scope
+// to ensure it completes at the required time.
+struct SourceCompressionTask
+{
+    friend class ScriptSource;
+    friend struct HelperThread;
+
+    // Thread performing the compression.
+    HelperThread* helperThread;
+
+  private:
+    // Context from the triggering thread. Don't use this off thread!
+    ExclusiveContext* cx;
+
+    ScriptSource* ss;
+
+    // Atomic flag to indicate to a helper thread that it should abort
+    // compression on the source.
+    mozilla::Atomic<bool, mozilla::Relaxed> abort_;
+
+    // Stores the result of the compression.
+    enum ResultType {
+        OOM,
+        Aborted,
+        Success
+    } result;
+
+    mozilla::Maybe<SharedImmutableString> resultString;
+
+  public:
+    explicit SourceCompressionTask(ExclusiveContext* cx)
+      : helperThread(nullptr)
+      , cx(cx)
+      , ss(nullptr)
+      , abort_(false)
+      , result(OOM)
+    {}
+
+    ~SourceCompressionTask()
+    {
+        complete();
+    }
+
+    ResultType work();
+    bool complete();
+    void abort() { abort_ = true; }
+    bool active() const { return !!ss; }
+    ScriptSource* source() { return ss; }
+};
+
+} /* namespace js */
+
+#endif /* vm_HelperThreads_h */